The demand for science, technology, engineering, and mathematics (STEM) graduates has increased significantly over the last decade. Because of the insufficient number of STEM graduates, this demand has not been met, resulting in a significant shortage of STEM workers in the US (President's Council of Advisors on Science and Technology, 2012). The first year of college has been identified as a critical period when attrition in STEM typically occurs (Chen, 2013), highlighting the need to study this period to understand why. In this study, the authors examined the impact of having an engaged mentor among a generally diverse sample of undergraduates who intend to major in a STEM discipline in which the first-year curriculum is particularly challenging and for whom the risk of attrition is high. Two cohorts of students were followed across their first year and assessed at 3 time points each. Having an engaged mentor at the start of the first year of college predicted two outcomes: (1) higher academic self-efficacy across the first year of college compared to students without a mentor, indicating that guidance and advice from engaged mentors positively impacts students' appraisal of their academic abilities and performance; and (2) a greater sense of belonging across the first year of college, suggesting that engagement with a mentor increases students' feelings of being welcomed and accepted by members of their new academic community.